Systems such as are briefly described above, are disclosed in U.S. Pat. Nos. 3,740,742 (Thompson), 3,810,172 (Burpee), 4,023,167 (Wahlstrom), 4,476,459 (Cooper et al.) and 4,531,117 (Nourse et al.). All such systems exploit a common feature, namely, that receivers for detecting the marker-produced signals are only activated during quiescent intervals between the transmitted bursts of RF. Accordingly, the much less intense signals produced by the markers are not masked by the much more intense transmitted bursts. The sensitive receivers used in the systems may, however, render the systems unduly prone to false alarms caused by radiation from other sources in the area, such as electrical motors, lights, radio and TV transmitting equipment and the like. Interference may also occur from nearby article surveillance systems, and in some cases even from transients or other spurious signals within the systems themselves. Thus, for example, as disclosed in U.S. Pat. No. 4,476,459 (Cooper et al.), some prior systems have attempted to avoid such false alarms by including detection circuits in which the rates of decay of the marker-produced signals are closely scrutinized. To further avoid interference between similar surveillance systems operating in the same area, it has also been known to hard-wire such systems together, thus ensuring synchronization of the transmitted pulses so that the transmitter produced pulses radiated by one system cannot occur during the quiescent intervals of the other system. Such interconnections have obvious drawbacks, and frequently cannot be used, particularly where the systems are to be installed in various retail stores within a single shopping mall.